1. Field of the Invention
The present invention relates to a resistor sensor input apparatus, connected to a resistor sensor via a signal line, detecting a resistance value in the resistor sensor by causing a current to flow through the resistor sensor via the signal line and transmitting the resistance value to a controlling apparatus or a monitoring apparatus.
2. Description of the Related Art
In an automatic controlling apparatus or the like, a resistor sensor is mostly used as a temperature detecting element for detecting various temperatures. The resistor can detect a temperature owing to its characteristic that the resistance value varies in accordance with the temperature. In general, a resistor sensor input apparatus having a reference current source is used to detect a resistance value of the resistor sensor.
A general resistor sensor input apparatus supplies a reference current to a resistor sensor, detects a voltage generated in the resistor sensor, amplifies the detected voltage to an analog voltage of a required level and finally converts the amplified analog voltage to a digital voltage.
FIG. 8 is a schematic diagram showing the general resistor sensor input apparatus.
Three signal lines 2a, 2b and 2c are connected to a resistor sensor 1 having a resistance R.sub.x. The signal line 2a is connected to a positive input terminal of a differential amplifier A2 in a resistor sensor input apparatus 3. The signal line 2b is connected through a reference resistance R.sub.0 to a negative input terminal of the differential amplifier A2 in the resistor sensor input apparatus 3. The signal line 2c is grounded in the resistor sensor input apparatus 3.
A constant current I.sub.ref1 supplied from a constant current circuit 4a flows to the ground via a line resistance R.sub.w of the signal line 2a, the resistance R.sub.x of the resistor sensor 1 and a line resistance R.sub.w of the signal line 2c. A constant current I.sub.ref2 supplied from another constant current circuit 4b flows to the ground via a line resistance R.sub.w of the signal line 2b and the line resistance R.sub.w of the signal line 2c.
Voltages (R.sub.w +R.sub.x +R.sub.w). I.sub.ref1 and (R.sub.0 +R.sub.w +R.sub.w). I.sub.ref2 are respectively applied to the positive and negative terminals of the differential amplifier A2. Accordingly, an output voltage E.sub.d from the differential amplifier A2 is obtained by the following equation: EQU E.sub.d =(2R.sub.w +R.sub.x).multidot.I.sub.ref1 -(2R.sub.w +R.sub.0).multidot.I.sub.ref2.
If the constant currents I.sub.ref1 and I.sub.ref2 are equal to each other, the following output voltage E.sub.d will be obtained: EQU E.sub.d =(R.sub.x -R.sub.0).multidot.I.sub.ref1
Since the value of the reference resistance R.sub.0 is known, the value of the resistance R.sub.x of the resistor sensor 1 is not influenced by the line resistances R.sub.w of the signal lines 2a to 2c connecting the resistor sensor input apparatus 3 and the resistor sensor 1, as represented by the following equation (1): EQU R.sub.x (E.sub.d /I.sub.ref1)+R.sub.0 ( 1)
FIG. 9 shows an another type of resistor sensor input apparatus, which can be connected to a number of resistor sensors 1. The resistor sensor input apparatus of this type has constant current circuits 4a and 4b and a reference resistance R.sub.0 for each of the resistor sensors 1.
Even if a monitoring apparatus or a control apparatus connected to the resistor sensor input apparatus receive a plurality of resistance values R.sub.x of the resistor sensors 1, it cannot process the values simultaneously. Therefore, the resistances R.sub.x of the resistor sensors 1 are successively read in a time-divisional manner. For this reason, a pair of cooperating switches S are interposed between a differential amplifier A2 and the signal lines 2a and 2b of each resistor sensor 1, so that only one differential amplifier A2 suffices. A capacitor C is connected between common terminals of the pair of the switches S.
Normally, the switches S are turned to the sides of the signal lines 2a and 2b, so that a voltage across the signal lines 2a and 2b is held in the capacitor C. The switches S in the respective resistor sensors 1 are successively turned to the side of the differential amplifier A2 for a very short period of time, so that the line voltage held in the capacitor C can be applied to the differential amplifier A2.
However, the resistor sensor input apparatuses shown in FIGS. 8 and 9 have the following drawbacks to be overcome.
First, in the resistor sensor input apparatus shown in FIG. 8, it is necessary that the constant currents I.sub.ref1 and I.sub.ref2, output from the constant current circuits 4a and 4b, precisely coincide with each other, in order to detect the resistance R.sub.x with high accuracy. Actually, however, even if the constant current circuits 4a and 4b have the same specification, they cannot be precisely the same, due to the difference in accuracy of the elements of the circuits 4a and 4b. A relative error and an absolute error in the constant currents I.sub.ref1 and I.sub.ref2 directly result in an error in the detection accuracy of the resistance R.sub.x.
Secondly, a constant current circuit which outputs a precisely constant current is expensive. If two expensive constant current circuits 4a and 4b are incorporated in a resistor sensor input apparatus, the manufacturing cost will be considerably increased.
Even if expensive constant current circuits 4a and 4b are employed, when they are actually incorporated in an apparatus, the values of the constant currents I.sub.ref1 and I.sub.ref2 of the circuits 4a and 4b must be adjusted in advance so as to coincide with each other. In addition, since it is necessary to precisely set the values of the constant currents I.sub.ref1 and I.sub.ref2 as indicated by the equation (1), the values must be re-adjusted periodically so as to coincide with the set value.
In the case of the resistor sensor input apparatus, as shown in FIG. 9, for detecting the values of the resistances R.sub.x of the plurality of resistor sensors 1, since two constant current circuits 4a and 4b are required in each of the resistor sensors 1, the input apparatus suffers from substantially the same problem as in the case of detecting the value of the resistance R.sub.x of one resistor sensor 1 as shown in FIG. 8.